Currently, there are many kinds of methods for detecting noise of FM receiver in the prior art. These methods include an estimation manner to be served as a detection reference standard such as a received signal strength indication (RSSI), a manner for measuring the abrupt change of the time domain shape after observing the time domain shape of audio signal in high-pass filter (HPF) or low-pass filter (LPF), and a manner for detecting the noise by determining whether the signal level of L (left)-R (right) audio signal is greater than the signal level of L+R audio signal.
As shown in FIG. 2, a pilot tone is inserted to 19 kHz of baseband domain with respect to FM radio signal. FIG. 2 illustrates the characteristic of band domain in FM radio signal. FIG. 1 shows a conventional FM digital receiver.
As shown in FIG. 1, FM digital receiver includes a radio frequency to baseband converter 10 for detecting the baseband signal of the radio signal received from the antenna, an analog-to-digital converter (ADC) 20 for converting the output signal of radio frequency to baseband converter 10 into the digital signal, an automatic gain control (AGC) 50 for controlling the gain of radio frequency to baseband converter 10 after receiving the received data from ADC 20, received signal strength indication 60 for indicating RSSI based on gain control value of AGC 50, FM demodulator 30 for demodulating the output signal of ADC 20, a pilot carrier sync detector 40 for controlling the frequency synchronization of FM demodulator 30 after the pilot carrier sync signal in FM demodulator 30 is detected, a noise reduction 70 for controlling noise decay of the output signal of the received signal strength indication 60, and a multiplex (MPX) decoder 80 for decoding the modulated data from the FM demodulator 30 for outputting L and R audio signal after the noise reduction 70 reduces the noise.
The FM digital receiver performs FM modulation and detects the pilot tone for executing frequency sync to separate L+R audio signal from L−R audio signal in order to generate L and R audio signal.
Meanwhile, the feature of FM receiving/transmitting is that the L−R audio signal is insensitive to noise. When an audio noise reduction is employed, received signal strength indication (RSSI) is adopted to reduce noise of L−R audio signal, which is termed as Stereo Noise Cancellation (SNC) technique or Soft Stereo Blending (SBL).
Furthermore, while employing received signal strength indication (RSSI), high frequency audio is processed according to high frequency cut based on RSSI, or L−R, L+R, L or R audio signals is processed by a mute processing. However, such the above-mentioned manners of noise reduction will result in distortion of audio signal. Therefore, it is required to precisely determine whether noise exists or not before SNC, HCC (high cut control), and Mute are adopted, which are described as follows.
(1) SNC type—Stereo signal is converted to Mono signal.
(2) HCC type—High frequency signal is reduced.
(3) Mute type—Audio signal is processed by mute processing.
According to the above-mentioned descriptions, RSSI is employed by FM receiver. Because FM receiver has intrinsic thermal noise, signal-to-noise (SNR) is enlarged while RSSI is increased and signal-to-noise (SNR) is reduced while RSSI is decreased. However, the problems of employing RSSI are that the several noise patterns including multipath fading and impulse interference cannot be solved.
It is ineffective to measure the abrupt change of the time domain shape after observing the time domain shape of audio signal in high-pass filter (HPF) or low-pass filter (LPF). The concept of such the manner is that the abrupt change of basic shape of audio signal is detected and determines whether the noise exists when single tone audio signal is inputted. However, the audio signal is not only composed of single tone audio but a plurality of frequency bands. Further, the time domain shape is not standard type so that such the manner cannot be utilized.
While determining whether the signal level of L (left)-R (right) audio signal is greater than the signal level of L+R audio signal for the noise detection, which employs the feature of FM receiving/transmitting, the noise effect is gradually increased when frequency raises even if a flat noise, e.g. Additive White Gaussian Noise (AWGN), exists in full frequency band. That the signal level of L−R audio signal is greater than the signal level of L+R audio signal seldom is a rare situation. The reason is that audio signal tends to mono type, even the stereo type, so that “L” and “R” signals cannot be totally separated according to the audio intrinsic characteristic.
Therefore, although the enlarged noise can be detected when the signal level of L (left)-R (right) audio signal is greater than the signal level of L+R audio signal, however, the problem is that the enlarged noise cannot be identified before user listens to the enlarged noise which user feels uncomfortable.